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The results of investigations of the effect of long-term service of steel on the characteristics of its static cyclic crack resistance are presented. The kinetic diagrams of fatigue failure for the whole range of fatigue crack rates (from the rates close to the threshold of 10 -m m/cycle to 10 -3 m/cycle) are analyzed. it is shown that acceleration of fatigue crack growth as a result of a changc in the state of the metal in service is most pronounced in the third and threshold regions of the effective kinetic diagrams of fatigue failure. The observed regularities are explained by structural transformations in the metal of a steam line under the effect of service factors.Service specifications of steam lines of steam power plants specify operating temperatures (up to 550~ steam pressure in a steam line (up to 14 MPa), and the content of impurities that are active in term of corrosion in a working fluid. The joint effect of these factors for a prolonged time causes structural changes in the metal of the steam line, the degradation of its properties [I], and, as a consequence, damages. In a critical case uncontrollable failure of the weakest member occurs.It is well known that the action of compressed steam and its own weight in the walls of a steam pipe result in the development of a complex stressed state [2]. Insignificant pressure differentials in service or shutdowns and start-ups dictated by the technology cause conditions that correspond to multicycle low-amplitude fatigue with a high asymmetry of the loading cycle or to high-amplitude fatigue with a small number of cycles. Therefore, the failure of a steam line can be due to both a fatigue crack growth and a quasistatic one.The aim of the work is to establish the regularities of the effect of changes in a metal of a steam line as a result of its service on the characteristics of its static and cyclic crack resistance. Material and ProcedureWe investigated a low-carbon 12KhlMF steel which is widely used in the production of steam pipes of various steam power plants (the time of operation xop was from 48.103 to 19. 104h). Mechanical characteristics were determined by tensile testing cylindrical specimens 5 mm in diameter cut from a wall of a tube in the C-R direction. The static crack resistance J1c was established by four-point bend-testing prismatic C-R specimens 22 x 15 mm in section with a one-side edge notch following the standards [3]. Using the data of cantilever and four-point bend tests at R = 0, v = 0.1-10Hz we constructed kinetic diagrams of fatigue failure (KDFF) from a nominal span and a critical one [4]. The effect of fatigue crack closure was estimated by a familiar procedure [5]. The gain in crack length was measured with an optical microscope with a resolution of 0.01 mm or calculated from the change in compliance of a specimen. Taking into account the high plasticity of the steel, the growth of a fatigue crack in the high-amplitude region of loading (the third region of the KDFF) was described by a graphic dependence da/dN-J in logarithmic co...

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Power, petroleum processing, and chemical engineering equipment is often used under conditions of the simultaneous action of high temperatures, hydrogenating media, and mechanical loadings, which leads to a degradation of the structure and changes in the properties of metal [ 1 ]. The aim of the present work is to develop the method for fast aging of steels under the action of a high temperature and gas-like hydrogen, which should enable one to find correlation relations between the technological parameters and kinetics of structural degradation of structural elements.In our reasoning, we proceeded from the nonequivalence of the equilibrium solubility and diffusion mobility of hydrogen in a metal at different temperatures. As the temperature decreases, the diffusion processes are retarded (the diffusion coefficient of hydrogen in iron at 100 and 570~ is, respectively, 0.44 and 2.0 m2/sec [2]) and the equilibrium solubility of hydrogen in iron considerably drops (10 -6 and 10 -5 m2/kg, respectively, [3]). The proposed method enables one to record the equilibrium concentration of hydrogen at 100~ in the bulk of the specimen that is typical of a temperature of 570~ This supersaturation favors the active displacement of hydrogen to the nearest free (interior or exterior) surfaces. By leaving a solid solution, excess hydrogen either emerges from the metal or tries to settle on phase boundaries or defects. In the process of migration, it redistributes atoms of alloying elements [4]. Therefore, an abrupt cooling of a hydrogenated metal must, on the one hand, increase local stresses due to an excessive concentration of hydrogen and, on the other, intensify diffusion of alloying elements, i.e., favor the transformation of its structure.A prismatic specimen of 180 x 22 x 15 mm was placed in a hermetic chamber, which was filled out with a gas-like hydrogen under a pressure of 0.3 MPa. The ends of specimens were specially not clamped to avoid the appearance of temperature stresses. We performed heating up to 570~ with a rate of -3~ by passing an electric alternating current of -1.8 kA with a frequency of 50Hz. The temperature in the working section was maintained with an accuracy of + 5~ by using an automatic control system of a signal from a thermocouple fixed in the middle of the specimen. The specimen was held at 570~ for an hour to obtain a concentration of hydrogen across the whole thickness at this temperature. Thereafter, we stopped heating and abruptly cooled the specimen (with a rate of -2~ to a temperature of 100~ In the process of decreasing the temperature, heat was removed through current-supply blocks, which were intensively cooled with running water under a pressure of -0.2 MPa. The blocks were made hollow, which allowed us to use them both for heating and cooling of the specimen. In addition, to decrease the temperature gradient across the section as much as possible and to eliminate the appearance of temperature stresses, the distance between them is much greater than sizes of the cross section of the specim...